The targeting of cell surface proteins to apical or basolateral membranes of epithelial cells is crucial for proper renal function. Recent studies have suggested an important role for protein-protein interaction domains in this polarized targeting of proteins. Much of this work has focused on the role of PSD-95IDlg/ZO-1 (PDZ) domain proteins. In the brain, PDZ domain proteins bind to the carboxy-terminus of channels as well as receptors and cluster these proteins in synapses. PDZ domain proteins are likely to play a similar role in the targeting of cell surface proteins in epithelia. In Caenorhabditis elegans, three genes lin-2, lin-7 and lin-10, are important for the basolateral targeting of the worm EGF-Receptor. Our group has identified mammalian homologues of these PDZ domain proteins and detected a stable complex of mLin-2 and mLin-7 at the basolateral surface of renal epithelia. The PDZ domain of mammalian Lin-7 can bind to the carboxy-terminal tail of the betaine gamma amino butyric acid transporter-i (BGT-l) and may have an important role in the retention of BGT-1 at the basolateral surface of renal epithelia. However the exact mechanism that targets mLin-7 to the basolateral surface is unclear. Recent studies indicate that binding to mLin-2 may have an important role in this process but additional proteins that bind to the amino-terminus of mLin-7 may also be important. In this proposal we will focus on two of these newly identified mLin-7 binding proteins that we call Pals 1 and Pals2. Like mLin-2 these proteins are composed almost exclusively of protein-protein interaction domains including PDZ, Src Homology 3 and Guanylate Kinase domains with variable amino-terminal regions. We have also identified a new domain called the L27 domain that is repeated in these proteins and mediates the interaction with mLin-7. We have demonstrated differential targeting of Pals 1 and Pals2 in kidney epithelial cells and have begun to characterize their association with distinct binding partners. We hypothesize that these distinct binding partners control the differential targeting of the Pals proteins and affect the function of targeted proteins such as BGT-l. To test these hypotheses we will examine the role of the different protein-protein interaction domains on cell surface targeting of these proteins and identify new proteins that associate with Palsi and Pals2. We will also examine the importance of mLin-7 and its binding partners on the targeting and activity of the BGT- 1 transporter. This work will allow new insights into the function of the mLin-7 system in renal epithelia and increase our understanding of the basic mechanisms underlying protein targeting in polarized cells.